This invention relates to classifiers for particulate material and, more particularly, to classifiers of the type wherein particles are distributed to various collection points by entrainment in a moving current of air.
Representative prior art includes the following U.S. Pat. Nos.: Rice 197,897; Frink 248,584; Davidson 527,835; Edison 775,965; Stromborg 1,042,836; Stebbins 1,759,959; Stebbins 1,834,981; Cowher 2,978,103; and Murphy 3,385,436.
A variety of industries require the use of classifiers for particulate material in their operations. Most particulate material classifiers fall into either of two general categories. The first general category employs the use of screens which, in many instances, are sufficient for a particular type of job. However, where a high throughput of material to be classified is required, screen type classifiers are impractical.
The second basic type of classifier is one which classifies by entraining particles in an air stream. Heavier particles are less likely to be substantially entrained than lighter particles and classification is thus achieved by taking advantage of this fact. Air stream classifiers have the advantage over screen classifiers of handling far greater amounts of material.
However, most air stream classifiers in use today are not without their disadvantages. In many instances, the particulate material to be treated will contain substantial fines that will not drop out of the air stream at the endmost classifying stage. As a result, in many instances, such fines are discharged into the environment with the exhaust from the air stream thereby causing an air pollution problem.
Attempts have been made to eliminate the air pollution problem by recirculating the air. The above identified Murphy patent is representative of this approach. However, where the particulate material is relatively hard as, for example, sand, the fines mentioned above are recirculated through an air impelling device and may cause rapid deterioration of the same due to abrasive contact therewith. Thus, such equipment requires substantial maintenance.